Observation and branching fraction measurement of the decay ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-}$

The decay ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-}$ is observed using a proton-proton collision data sample collected at center-of-mass energy $\sqrt{s}=13$ TeV with the LHCb detector, corresponding to an integrated luminosity of 5.5 fb${}^{-1}$. This process is mediated by the $s\to u\bar{u}d$ quark-level transition, where the $b$ quark in the ${\mathrm{\Xi }}_b^{-}$ baryon is a spectator in the decay. Averaging the results obtained using the two ${\mathrm{\Lambda }}_b^0$ decay modes, ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$ and ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$, the relative production ratio is measured to be $(f_{{\mathrm{\Xi }}_b^{-}}\mathrm{/}{f}_{{\mathrm{\Lambda }}_b^0})\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})=(7.3\pm 0.8\pm 0.6)\times {10}^{-4}$. Here the uncertainties are statistical and systematic, respectively, and $f_{{\mathrm{\Xi }}_b^{-}}(f_{{\mathrm{\Lambda }}_b^0})$ is the fragmentation fraction for a $b$ quark into a ${\mathrm{\Xi }}_b^{-}$ (${\mathrm{\Lambda }}_b^0$) baryon. Using an independent measurement of $f_{{\mathrm{\Xi }}_b^{-}}\mathrm{/}{f}_{{\mathrm{\Lambda }}_b^0}$, the branching fraction $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})=(0.89\pm 0.10\pm 0.07\pm 0.29)\mathrm{\%}$ is obtained, where the last uncertainty is due to the assumed SU(3) flavor symmetry in the determination of $f_{{\mathrm{\Xi }}_b^{-}}\mathrm{/}{f}_{{\mathrm{\Lambda }}_b^0}$.